Hyperpolarization-activated Cyclic Nucleotide-gated Channel Blocker Research Articles

α2A-Adrenoceptors Strengthen Working Memory Networks by Inhibiting cAMP-HCN Channel Signaling in Prefrontal Cortex

Spatial working memory (WM; i.e., "scratchpad" memory) is constantly updated to guide behavior based on representational knowledge of spatial position. It is maintained by spatially tuned, recurrent excitation within networks of prefrontal cortical (PFC) neurons, evident during delay periods in WM tasks. Stimulation of postsynaptic alpha2A adrenoceptors (alpha2A-ARs) is critical for WM. We report that alpha2A-AR stimulation strengthens WM through inhibition of cAMP, closing Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels and strengthening the functional connectivity of PFC networks. Ultrastructurally, HCN channels and alpha2A-ARs were colocalized in dendritic spines in PFC. In electrophysiological studies, either alpha2A-AR stimulation, cAMP inhibition or HCN channel blockade enhanced spatially tuned delay-related firing of PFC neurons. Conversely, delay-related network firing collapsed under conditions of excessive cAMP. In behavioral studies, either blockade or knockdown of HCN1 channels in PFC improved WM performance. These data reveal a powerful mechanism for rapidly altering the strength of WM networks in PFC.

Pacemaking by HCN Channels Requires Interaction with Phosphoinositides

Hyperpolarization-activated, cyclic-nucleotide-gated (HCN) channels mediate the depolarizing cation current (termed I(h) or I(f)) that initiates spontaneous rhythmic activity in heart and brain. This function critically depends on the reliable opening of HCN channels in the subthreshold voltage-range. Here we show that activation of HCN channels at physiologically relevant voltages requires interaction with phosphoinositides such as phosphatidylinositol-4,5-bisphosphate (PIP(2)). PIP(2) acts as a ligand that allosterically opens HCN channels by shifting voltage-dependent channel activation approximately 20 mV toward depolarized potentials. Allosteric gating by PIP(2) occurs in all HCN subtypes and is independent of the action of cyclic nucleotides. In CNS neurons and cardiomyocytes, enzymatic degradation of phospholipids results in reduced channel activation and slowing of the spontaneous firing rate. These results demonstrate that gating by phospholipids is essential for the pacemaking activity of HCN channels in cardiac and neuronal rhythmogenesis.

Action of the hyperpolarization-activated current blocker ZD7288 in rat dorsal root ganglion neurons

Peripheral and spinal hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play a key role in neuropathic pain by regulating neuronal excitability. HCN channels are expressed in the ventral–lateral periaqueductal gray (vlPAG), a region that is important for pain modulation. However, the role of vlPAG HCN channels in neuropathic pain remains poorly understood. In the present study, we investigated the impact of changes to vlPAG HCN channels on neural activity in neuropathic pain. First, sciatic nerve chronic constriction injury (CCI) was established as a neuropathic pain model. Then, changes in HCN channels and their influence on vlPAG neuronal activity were detected. Our results indicate that after CCI surgery the following changes occur in vlPAG neurons: the expression of HCN1 and HCN2 channels is increased, the amplitude of the hyperpolarization-activated current (Ih) is augmented and its activation curve is shifted to more positive potentials and there is an increase in the frequency of action potential (AP) firing and spontaneous EPSCs that is attenuated by ZD7288, a HCN channel blocker. In addition, forskolin, which can elevate intracellular cAMP, mimics the CCI induced changes in neuronal excitability in the vlPAG. The effects of forskolin were also reversed by ZD7288. Taken together, the present data indicate an important role for HCN channels in the vlPAG in neuropathic pain.